Not receiving external Read receipts

I have a single exchange server is users are not receiving external Read receipts. The users do receive delivery receipts.
Internal - internal recieve both types.
We do use an hosted spam solution but I have checked the quarantines and this is not blocking them.
The issue seemed to occur when we change ISP. Everything was fully functioning previously. To compensate for this we started using a payed for smtp gateway. Still not getting receiving Read receipts although other customers using this smtp gateway do receive their receipts.

I hope this explains all and that someone can help

Kind Regards
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Do you receive NDR notifications?
Do you filter messages with blank senders in the "Sender filtering" in the "Message Delivery" properties?

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pbraneAuthor Commented:
We don't filter blank messages in 'sender filtering' in message delivery properties. We don't receive any NDR messages just a message saying the following:

This is an automatically generated Delivery Status Notification.

Your message has been successfully relayed to the following recipients, but the requested delivery status notifications may not be generated by the destination.
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